Fitch graphs have been introduced as a model of xenology relationships in phylogenomics.
Directed Fitch graphs G=(X, E) are di-graphs that are explained by {0, 1}-edge-labeled
rooted trees with leaf set X: there is an arc xy∈ E if and only if the unique path in T that
connects the least common ancestor lca (x, y) of x and y with y contains at least one edge
with label 1. In this contribution, we consider the undirected version of Fitch's xenology
relation, in which x and y are xenologs if and only if the unique path between x and y in T …

… The undirected Fitch graph is therefore the underlying undirected graph of the directed Fitch
graph, ie, it is obtained from the directed version by ignoring the direction of the arcs. … Hence,
only three of the four possible undirected graphs on three vertices can be realized, while K1
⊍K2 is not an undirected Fitch graph. By heredity, K1 ⊍ K2 is therefore a forbidden induced
subgraph for the class of undirected Fitch graphs. Finally, it is well known that the class of …